1. Field of the Invention
The present invention relates to using carbon nanotubes in a thermal interface to increase the heat flow between two surfaces, and, in particular, to using arrays of aligned or densely packed carbon nanotubes to improve thermal contact in a reusable thermal interface, such as a thermal interface for use in a thermal switch.
2. Description of the Related Art
There are many applications that benefit from increasing heat flow between two surfaces. Heat transport is accomplished by phonons, quantized modes of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The flow of heat between two surfaces depends on how well these phonons couple from one surface to the other, the inverse of which is described by the thermal contact resistance. The thermal contact resistance depends how well the two surfaces are bonded together. In many applications, however, the surfaces could be of different materials or fabricated with temperature restrictions, such as for electronic components, which can reduce the quality of the bond between the surfaces, and therefore increase the thermal resistance
In many applications the surfaces can not be bonded for functional reasons. For example, in a device that is subjected to alternating heating and cooling, the device may be connected thermally to a radiator to dissipate excess heat during a heating time interval, but disconnected to preserve heat, or thermally connected to a heat source, during a cooling interval. For example, such control of heating and cooling is required on a spacecraft, to balance heating from power dissipation within the spacecraft and from exposure to the sun, and to balance cooling during an eclipse while in the shadow of earth, or another planet. A thermal switch can be used to connect the device to a radiator to cool the device during a heating interval and to disconnect the device from the radiator to preserve heat during a cooling interval. The thermal switch may include two plates that separate to reduce heat flow to the radiator and that come into thermal contact to allow heat flow to the radiator.
When separate surfaces are brought into contact, heat flow between the surfaces, per unit area, depends, among other factors, on the fraction of that area in contact that allows phonons to couple from one surface to the other. One way to increase the area in contact is to highly polish the surfaces and ensure they are absolutely flat. However, even highly polished surfaces are rough at some length scale and the fractional area of contact is always less than one. In some approaches, the two surfaces are forced together under pressure, which increases the fractional area of contact. However, this approach is of limited effectiveness when the surfaces are made of rigid materials, such as copper or diamond, that do not deform sufficiently under the pressures that can be applied in practical circumstances.
Heat flow can occur not only at the contact areas, but also across the gap between the surfaces, via radiative heat transfer, thermal conduction or convection in the interstitial medium. However, there is no conductive or convective transfer of heat in vacuum.
In some approaches, the gaps are filled with an epoxy that permanently binds to the surfaces of both surfaces. The epoxy fills the gaps and provides a solid medium for the transfer of heat. Using this approach, the heat flow is controlled by the conductivity of the epoxy. The thermal conductivity of the epoxy can be increased by including conductive materials in the epoxy, such as metallic or diamond particles, carbon or graphite fibers. Graphite is a layered structure of carbon that is very thermally conductive in the plane of each layer. While suitable for many purposes, the use of an epoxy is unsuitable as a reusable thermal interface in which the two surfaces separate under some circumstances, such as in a thermal switch.
In some approaches, thermally conductive grease is applied to the contact surface of one or both surfaces. While grease allows the surfaces to be separated, adhesion and flow properties of the grease generally involve relatively large separation of the surfaces to break thermal contact. Grease may not be practical for devices that repeatedly break and establish thermal contact or in small devices such as micro-electromechanical (MEMS) devices.
Other highly conductive materials also have limitations with respect to utility as a thermal interface in a thermal switch. Diamond is known to have very high thermal conductivity. However, diamond is very rigid and does not conform to fill gaps in a surface; and it is very expensive. Thermal conductivity varies with temperature and isotopic purity from about 600 to about 2600 Watts per meter per Kelvin (W/m·K). Isotopically pure diamond produced by chemical vapor deposition (CVD) is brittle.
Carbon fiber, such as K-1100, is a material known to have relatively high thermal conductivity, but not as high as diamond. Commercially available carbon fibers are made from pitch or Polyacrylonitrile (PAN) precursor material and drawn onto fiber tow. Pitch fibers are treated to form graphite structures (“graphitized”) by heating to high temperatures, near 3000° C. Overall thermal conductivity can be as high as 200 W/m·K.
Some thermal interfaces use carbon fibers, which can achieve thermal conductivities of 200 W/m·K along the fiber direction, arranged as a velvet on one or both surfaces. In some approaches, the carbon fibers are saturated with a phase change material (PCM) to conform to an uneven surface. A PCM changes from solid to liquid near some operating temperatures. In some approaches, the carbon fibers of the velvet are held erect in an epoxy or PCM on one surface and protrude above the epoxy or PCM to contact the other surface. For some applications carbon fibers can not be packed densely enough to provide sufficient thermal conductivity. Furthermore, carbon fibers are subject to bending and breakage, and may not be suitable for a thermal interface that is frequently reused or exposed to excess pressure.
Carbon nanotubes are large carbon molecules that are each akin to a graphite sheet rolled into a closed cylinder with a diameter of a few to tens of nanometers (nm, 1 nm=10−9 meters, m) and lengths of hundreds of nanometers to several micrometers (μm, 1 μm=10−6 m) or more. One or both ends are capped with a hemisphere of carbon atoms. Carbon nanotubes can be formed having one wall (single walled nanotubes, SWNT) or multiple co-axial walls (multiple walled nanotubes, MWNT). The thermal conductivity along the axis of a SWNT is higher than the thermal conductivity of diamond; SWNT thermal conductivity has theoretical values ranging from about 6,600 W/m·K to 37,000 W/m·K. A single MWNT has been measured to have a thermal conductivity of about 3000 W/m·K. Carbon nanotubes are strong and ductile and can compress under pressure without breaking and with less bending than carbon fibers. However, arrays of aligned carbon nanotubes in a configuration to provide thermal and mechanical advantages for a thermal interface are difficult to achieve.
In some approaches, carbon nanotubes are included in a thermal interface, alone or on the tips of larger carbon fibers arranged in a velvet (see for example, Timothy R. Knowles and Christopher L. Seaman, United States Patent Application Publication US 2002/0100581, Aug. 1, 2002, hereinafter “Knowles”). However, the nanotubes in Knowles are not densely packed. Packing ratios only up to about 50% are proposed. Therefore thermal conductivity of these interfaces does not approach the conductivity along the axis of nanotubes as closely as would more densely packed nanotubes. Knowles shows the construction of mops of tangled MWNT that have diameters of about 1 μm, or powders of non-continuous carbon nanotubes, deposited on the tips of carbon fibers with diameters greater than 3 μm. These structures can not achieve the thermal conductivity of MWNT between the two surfaces because heat has to pass through many different nanotubes and then through the lower conductivity carbon fiber.
Clearly, there is a need for a reusable thermal interface that increases heat flow between two surfaces over the heat flow attainable from the two surfaces in mechanical contact, and which does not suffer the disadvantages of prior art approaches.